Anyone who has ever had to clean a bathroom knows that certain surfaces tend rather quickly and easily to stain, soil, and accumulate mineral deposits and other undesirable build-ups.
In particular, porcelain surfaces, such as toilet bowls and sink basins, have a strong tendency to develop brown, orange, and/or yellow stains. These unsightly-colored stains are caused by metal complexes formed from high oxidation state metal ions, such as iron(III), manganese(III), and manganese(IV), which are typically present in the water supply, or which originate from other sources.
The removal of unsightly-colored stains on ceramic surfaces poses a difficult problem, especially with respect to porcelain toilet bowl cleaning. High oxidation state iron and manganese metals form kinetically inert (as opposed to labile) coordination complexes with the oxide or hydroxide ligand sites on the ceramic surface. Covalent bonding tightly binds the high valence state complexes to the surface. Unlike rust or iron oxide scales, such as FeO, Fe.sub.2 O.sub.3, and Fe.sub.3 O.sub.4, found on metal surfaces, these complexes are difficult to remove, even with mechanical abrasive action. Consequently, the complexes persist as the unsightly brown, orange, and/or yellow stains readily visible on the interior of porcelain toilet bowls.
As used hereafter, "kinetically inert metal coordination complex stains" refers to the complexes formed between high valence state metals, including at least iron and manganese, and free oxide or hydroxide ligand sites on surfaces, including at least ceramic and porcelain surfaces. One of ordinary skill in the art will recognize that kinetically inert metal coordination complex stains are distinct and altogether different from rust or iron oxide scales.
Early attempts to solve the above-described problem involved the use of a soluble ferrous salt, such as ferrous chloride, ferrous sulfate or ferrous nitrate, in an acidic cleaning solution comprised of phosphoric acid, hydrochloric acid, and mixtures thereof, as described in U.S. Pat. No. 3,173,875 (the '875 patent). The addition of the ferrous salt was found to increase the effectiveness of the solution in removing iron stains from porcelain surfaces. This patent, however, does not suggest using a reducing agent, such as isoascorbic acid, to remove kinetically inert metal coordination complex stains.
A drawback to the composition of the '875 patent lies in its phosphoric acid component. In recent years, phosphates have been recognized and perceived as being ecologically unsound. In addition, consumer demand for environment-friendly products continues to increase. It is desirable, therefore, to provide a hard surface cleaning composition free from phosphate-based compounds.
U.S. Pat. No. 4,828,743 (the '743 patent) discloses the use of ferrous ions in a non-phosphate cleaning composition for removing rust from toilet bowls, sinks, tubs, tiles, and the like. The ferrous ions were found to enhance the rust removing capability of the composition. To promote storage stability, an oxidation inhibiting substance, such as ascorbic acid or erythorbic acid, may be added to the composition to prevent oxidation of the ferrous ions. This oxidation inhibiting substance, though, does not appear to contribute to the rust removing property of the composition.
Unlike the present invention, however, the cleaning composition in the '743 patent fails to suggest a direct role for a reducing agent, such as isoascorbic acid, in the removal of kinetically inert metal coordination complex stains.
Various other additives have been proposed to improve the ability of acidic compositions to remove undesirable build-ups on hard surfaces. For example, to improve the ability of a composition to remove metal oxides and/or rust, the use of additives is discussed in U.S. Pat. No. 5,078,894, U.S. Pat. No. 5,587,142, and U.S. Pat. No. 4,477,285.
Both U.S. Pat. No. 5,078,894 (the '894 patent) and U.S. Pat. No. 5,587,142 (the '142 patent) disclose the use of a reducing agent in an acidic composition for removing metal oxides, particularly iron oxides, from various hard surfaces. The preferred reducing agent in those patents is sulfur-based. Non-sulfurbased reducing agents, such as ascorbic acid and hydroxylamine hydrochloride, are disclosed as comparative examples, but the data suggests that, relative to the sulfurbased reducing agents, these compounds are ineffective at ambient temperature for removing metal oxides. The preferred acid in those patents is a diphosphonic acid or polyphosphonic acid.
Unlike the present invention, however, the '894 patent and the '142 patent do not address the removal of kinetically inert metal coordination complexes from ceramic surfaces, such as porcelain and glass. Furthermore, the use of a phosphate-based acid in the compositions in those patents raises the same ecological concerns discussed above.
U.S. Pat. No. 4,477,285 (the '285 patent) discloses a two component composition for treating a surface susceptible to oxidation, such as wood, plastic, ceramic, or metal. The composition consists of a particulate abrasive material and an ascorbic-type reducing compound. The abrasive material removes paint, surface finishes, rust, or other oxidized layers coated on or integral with the surface, when the composition is rubbed thereon. The reducing compound then functions primarily as a protective agent to accept or intercept oxidizers which would otherwise contact the abraded surface.
Although recognized as providing some rust removing-function due to their acidity, the ascorbic-type reducing compounds in the '285 patent function primarily as anti-oxidants, and not as reducing agents, to protect cleaned surfaces from oxidative degradation. Moreover, that patent fails to suggest an active role for a reducing agent, such as isoascorbic acid, in the removal of oxidized layers from hard surfaces.
The state of the art in bathroom cleaning solutions today utilizes strong acids, such as hydrochloric, sulfuric, phosphoric, and the like, in combination with a small quantity of surfactant, dye, and fragrance. Some of the products on the market utilize combinations of acids which improve performance. These aqueous acid solutions lower tap water pH down between about pH 1.0 and about pH 3.0, but as demonstrated below in the Comparative Tests, they only exhibit a fair effect, if any, in removing kinetically inert metal coordination complex stains, lime scale, and soap scum. Unlike the present invention, these bathroom cleaning solutions do not include a reducing agent component, such as isoascorbic acid, to accelerate stain removal. Furthermore, strong mineral acids, such as hydrochloric acid and sulfuric acid, can damage (e.g., cause erosion and/or pitting of) the porcelain surface, thereby causing the porcelain surface to soil more quickly.
A cleaning composition for ceramics, porcelain, glass, and other hard surfaces should remove at least lime scale, soap scum, soil, grease, and biofilm deposited thereon, in addition to removing kinetically inert metal coordination complex stains. Various acid-based compositions have been proposed to remove lime scale, soap scum, and grease from hard surface items, including bathroom surfaces, as disclosed in U.S. Pat. No. 5,192,460, U.S. Pat. No. 5,294,364, and U.S. Pat. No. 5,554,320. However, none of the compositions disclosed therein suggests any effectiveness in removing kinetically inert metal coordination complex stains from ceramics and other hard surfaces. Furthermore, none of these patents discloses or suggests the use of an additive, such as a reducing agent, to improve the stain removing ability of the composition.
The above discussion illustrates the need for a cleaning composition effective in removing kinetically inert metal coordination complex stains, lime scale, soap scum, soil, grease, biofilm, and other build-ups from ceramic, porcelain, glass, and other hard surfaces. Heretofore, no composition has been capable of achieving such cleaning functions.